1. Field of Invention
The present invention relates to ship hulls in general and in particular to a ship hull having a single hull with a wave piercing bow.
2. Description of Related Art
A ship moving in waves is subjected to vertical accelerations. A traditional bow, having transverse sections that are the widest at the deck, is typically lifted up by its volume when it passes through a wave crest, which then causes it to fall into the trough between waves. This periodic movement is commonly referred to as “pitching”, and in certain conditions it results in high amplitude and acceleration levels. In extreme cases, at certain combinations of ship's speed, hull length and wave height, the hull will emerge from, and then crash back into the water.
To prevent damage to the hull, the rules governing structural design of ships, call for a suitably strong, and therefore heavy, shell and its supporting structure. Structural strength requirement can be reduced under the construction rules only if the global loads (bending moments between the waves, referred to as sagging, or over the wave, referred to as hogging) and local loads (slamming) are reduced. In turn, these loads can only be lowered by reducing the level of hull's response to the waves. The benefits of reduced structural weight are significant. Firstly, the construction cost is reduced, as less material needs to be purchased and installed. Secondly, a lighter craft requires less power to achieve the required speed, further reducing the cost of building and operating such a vessel, as smaller engines need to be purchased and less fuel is consumed in service. Finally, reduced acceleration levels result in reducing damage to cargo and the incidence and severity of seasickness in crew and passengers. This directly and significantly increases the revenue-generating potential of ships.
Additionally, certain types of ships, such as passenger ferries or crew boats serving offshore installations, are often restricted in their operation by weather conditions due to the adverse effect of severe motions on crew and passengers. In heavy weather, speed may have to be reduced and course may have to be altered, increasing cost of operating a vessel. Also, in certain weather conditions, pitching motion may cause propellers to emerge and the hull to lose speed. Accelerating back to operating speed requires extra time and energy, and results in overall increase in fuel consumption and operating costs. Finally, in some cases, ships are not permitted to operate at all above certain sea states, present or forecasted.
The described problem has been addressed in the past by introduction of several different types of marine craft, specifically designed to reduce response to wave action and to minimize pitch amplitude and accelerations. One design solution has been to utilize multihull ships fitted with wave piercing bows and a third center bow above static waterline to prevent diving as a passive ride control. However, multihull ships have a relatively low payload to overall weight ratio, because of their heavy decks connecting relatively narrow hulls. Disadvantageously, multihull ships only work up to a certain wave height above which waves begin to hit the deck spanning the hulls, with often even worse effect than a conventional monohull. Such multihull ships are also commonly expensive to construct. There are also several methods of mitigating motions of monohulls in waves with active ride control devices, such as computer controlled articulated foils, however, these are invariably expensive and prone to damage.